Variable resistance device



0d; 1939. N. c. SCHELLENGER.

VARIABLE RESISTANCE DEVICE Filed Feb. 2, 1939 A c-107572 0 fic/zeZe INVENTOR.

BY, I

ATTORNEYS.

Patented Oct. 24, 1939 VARIABLE RESISTANCE DEVICE Newton 0. Schellenger,

Elkhart, Ind., assignor to Chicago Telephone Supply Company, Elkhart, Ind., a corporation of Indiana.

Application February 2, 1939, Serial No. 254,319

6 Claims.

This invention relates to a new and improved variable resistance device and more particularly to a device of this type in which the mechanical thrusts upon the parts are balanced inorder to minimize wear and to insure smooth mechanical and electrical operation.

Variable resistance devices are in general use in large numbers in installations where they are repeatedly operated and where noise-free opera- 10 tion is important. Among such uses of these devices are their'employment in radio receivers and transmitters and in public address systems. The devices are commonly utilized in such apparatus as gain or volume controls or tone controls. In such uses the devices are repeatedly adjusted to vary the resistance and the parts are subject to wear. In many instances the variable resistances are jointly operated with switch which puts an additional mechanical load upon :0 the moving parts of the devices and causes them to be operated whenever the apparatus is put into or out of operation. I

Where a variable resistance is used as a tone or volume control at a point in a circuit prior to any substantial audio amplification, it is essential that the variation in resistance by operation of the device be smooth electrically. Otherwise objectionable noises are introduced into the circuit. It is also essential that the contacts in the device be uniform and stable so that the unitintroduces nonoises caused by fluctuation in resistance when the device is in any adjusted position. Devices of this character as generally constructed comprise an arcuate resistance element together with a contact moved along the resistance by means of a rotary shaft. The moving contact is electrically connected to a fixed terminal by means of a brush or brushes connected 0 electrically to the movable contact and engaging a collector member which is electrically connected to the fixed terminal. Such a device is shown in my prior application, Serial No. 218,062,

flled July 8, 1938, and the present invention is in the nature of a modification of the device of that application.

The device comprises a non-conducting drive arm which is fixedly secured to the rotating shaft. The contact arm and transfer brushes extend from the under side of the drive arm and engage the resistance and collector ring respectively. Resilient connections between the contact and drive member and brush and drive member are necessary in order to give the proper contact pressure and to take care of surface inequalities of the members and of any variation of the axis of rotation from a position normal to the planes of the resistance and the collector ring.

In the usual forms of construction, the spring 5 pressures of the contact arm and brush cause unbalanced reactions against the shaft which thrusts the shaft laterally. Since there must be some slight play between the shaft and its bearing in order to permit the shaft to rotate easily, this lateral thrustcauses a tilting of the shaft and of the drive arm which interferes with the smooth operation of the device. The tilting causes limited contact areas and high contact pressures between the shaft and bushing which increases wear of the parts and makes for irregular mechanical action. Also, if the bore of the bushing is not entirely smooth there is a tendency for the shaft to ride over the irregularities with a jerky movement in operation of the device.

In the construction of the present invention the resistance contact and transfer brush are so designed and related as to give a substantially balanced thrust to the drive shaft. This reduces the lateral bearing pressures of the shaft to a very low amount. 'The end thrust bearings with the exception of the end of the bushing are normally made of rolled sheet metal and are quite smooth. The end of the bushing is easily made smooth during its manufacture. In view of the fact that the collector ring is located closer to the shaft than the resistance element and on a somewhat differentplane, this involves more than a location of the partsupon opposite sides of the shaft. It is necessary to so proportion the effective lever arms of the contacts and brushes and their spring pressures that the thrusts are balanced. For example, the effective lever arm of the transfer brush may be half that of the contact and its spring pressure on thecollector twice that of the contact on the resistance element. This has the desirable result of giving a higher contact pressure to the metal to metal engagement of thQDIUSh with the collector member.

The balanced pressure causes the drive member to tend to operate in a plane perpendicular to the axis of the shaft instead of causing it to tilt to a substantial extent as is the case in constructions Where all pressures are on one side of the shaft or where the pressures are out of balance to any considerable extent. In devices where the drive arm is formed from sheet insulation there is a tendency for the arm to bow due to tilting and unbalanced thrusts upon the arm. This is particularly true under conditions of high humidity which tend to cause a slight softening of the material. Further, the ears or other means used to secure the metal contact and brush to the drive arm are subjected to less strain in a balanced construction.

With the contact arms and brush arms upon opposite sides of the'shaft there is greater freedom to design them individually to better perform their functions without consideration as to relative clearances between them. The brushes may be made straight for a major portion of their length so as to give a maximum resistance to chattering. This construction eliminates as much springiness as possible in the direction in which the frictional force is applied at the end of the brush.

The contact arms and brush arms may be formed simultaneously from a single piece of sheet metal. By setting the forming die so that as nearly as complete balance between the brushes and resistance contacts is obtained as is prac-'- ticable, extremely close production tolerances on this balance can be maintained as any variation in stock thickness or temper usually affects the entire contact and brush member and therefore substantially uniformly influences all ofthe portions extending therefrom.

It is an object of the present invention to provide a new and improved variable resistance construction providing balanced and equalized thrusts upon the rotary shaft.

It is a further object to provide a construction of this character in which balancing thrusts are caus-d by locating element contact and current transfer brush contacts upon opposite sides of the shaft.

It is an additional object to provide a construction wherein the metal to metal current transfer contacts operate at substantially greater contact pressure than the resistance unit contacts.

It is also an object to provide a unit which is composed of but few and simple parts and adapted for commercial production and use.

Other and further objects will appear as the description proceeds.

I have shown a preferred embodiment of my invention in the accompanying drawing, in which- Figure 1 is a plan view of the device;

Figure 2 is a section taken on line 22 of Figure 1;

Figure 3 is a fragmentary view showing the drive member and contact and brush plate; and

Figure 4 is aview similar to Figure 3 but showing a modified form of construction.

The device comprises an insulating base II to which is secured the shaft supportingthimble or bushing l2. This bushing has an enlarged end l3 engaging the inner face of the base II and has a portion upset at l4 to hold a'washer l5 against the grounding plate l6 which in turn is held against the outer face of the base II. The shaft I1 is journalled in the bushing I2 and is provided with a C-washer I8 fitted in a groove ID in the shaft and. engaging he outer face of thebushing. The inner end of the shaft I1 is 'provided with a reduced extension 23 which passes through an opening 2| in the non-conducting drive plate 22 to cause that plate to rotate with the shaft. The reduced portion also passes through a similar opening in the stop plate 22 and its outer end is deformed as indicated at 23 to retain the parts in assembled relation.

The stop plate 24 is provided with upturned stop lugs 25 and 26 which are adapted to alternately engage opposite sides of the inwardly depressed stop portion 21 of the metallic housing 28. The stop plate 24 is also provided with the upturned switch operating lug 29 which is adapt-, ed to transiently engage a switch operating cam 30, shown in Figure 2 as extending from a switch assembly mounted in a housing 3| secured to the outer face 32 of the unit housing 28. The switch in itself forms no part of the present invention and need not be further described herein.

The fiat resistance element 33 is secured to the inner face of the base H, the resistance element being arcuate in form and located about the shaft H. The ends of this strip 33 are secured in place by the rivets 34 and 35 passing through solder lug terminals 35 and 31. The insulating washer 38 fits closely around the shaft l1 against the inner face of the enlarged portion l3 of the bushing l2. This washer 38 is of such size as to overlie the inner edge of the resistance strip 33 and thus prevent the intermediate portions of that strip from rising substantially from the base II. This washer 33 also serves to insulate the collector ring 39 from the bushing l2. This collector ring 39 has integrally connected thereto th'e extension 40, a portion of which sits in a recess 4| in the base H. The outer end 42 of this extension isformed as a solder lug for connection to an electrical circuit.

The combined contact and brush plate 45 is secured against the under face of the drive plate 22. In the form of construction shown in Figures 1 to 3 the connection is made by Z-shaped arms 46 which fit into notches 41 in the plate 22 and which have the opposite portion of the Z lying against the upper face of the plate. Further, as best shown in Figure 2, the member 45 is formed with a short tubular extruded portion 48 which fits into an opening 49 in the plate 22. The plate 45 is formed with the oppositely extending arcuate contact arms 50 and 5|. These arms terminate in overlapping rounded contact portions 52 and 53, respectively.

Upon the opposite side of the axis of the shaft H the plate 45 is provided with a pair of oppositely extendingbrush arms 54 and 55 having their rounded contact brush ends 56 and 51, respectively. It will be noted in the construction shown, that the current transfer brush arms 54 and 55 are shorter than the contact arms 54 and 5| and also are straight for the major portion of their length rather than being arcuate in form. This construction makes them somewhat stiil'er than the arcuate arms of the same size. The brush contact portions 58 and 51 are located upon slightly different radii from the axis of the shaft so that they make slightly separated lines of contact upon the collector ring 35. This is desirable in affording a uniform and permanently emcient contact, since each brush wears its own contact path to conform to its own shape. I

It will be noted that the point of engagement of the brush contacts 58 and 51 is substantially diametrically opposite across the axis of the shaft from the points of contact of the resistance contact members 52 and 53. It will also be apparent that in the construction shown the effective contact on the resistance is substantially twice as far from the axis of the shaft as the brush contacts on the collector ring. In order tary stamping of sheet metal so that variations in thickness, temper and other factors inherent to in the stock affect the long and short arms to equalize the pressure upon opposite sides of the axis of the unit it is therefore necessary to so form the contact arms and brush arms that the brushes have substantially double the pressure upon the collector ring that the contact arms have upon the resistance unit. An ex-. amination of Figure 2 will also make it apparent that the upper face of the resistance strip which is engaged by the contacts 52 and 53 is upon a lower plane relative to the body of the plate 45 than the collector ring 39. This difference in offset of the arms is also taken into account in providing the desired ratio of pressures. This pressure ratio fits in with the requirements of the device, since a higher contact pressure is desirable between metal surfaces than is desirable between movable contacts and a carbonaceous resistance element.

The form of construction shown in Figure 4 is similar to that shown in Figure 3 insofar as the operating portions of the contact plate are concerned. The plate 60, shown in Figure 4, is provided with the contact arms 6| and 62 and the brush arms 63 and '64. It is also ,provided with the upturned securing lugs 65 and 66 which pass up through lateral notches 51 and 68 in the drive plate 69. The free ends of the lugs 65 and 66 are then bent over against the upper face of the plate 69, as indicated in broken lines in Figure 4. This permanently secures the plate 60 to the drive member 69 and differs in that regard from the method of securing the plate 45 to the drive member 22 as shown in Figure 3, where the parts are readily detachable.

While I have shown a'preferred embodiment of my invention, it is to be taken as illustrative only, as it is capable of change and modification to meet differing conditions and requirements and I contemplate such variations as come within the spirit and scope of the appended claims.

I claim:

1. In a variable resistance device: a resistance element; a metal collector member; a contactor having long and short spring arms sprung in the same direction away from a medial attaching portion lying between the inner ends of the arms; a carrier for the contactor; means for connecting the medial attaching portion of the contactor to the carrier; said arms being so proportioned that the thrusts of which the long and short arms are capable are balanced about an axis passing through the medial portion of the contactor substantially medially between the inner ends of the long and short arms so that the unit pressure on the metal collector member is higher than the unit pressure on the resistance element; and the contactor being a uniequally.

2. In a variable resistance device: a base; an operating shaft rotatable with respect to the base; an arcuate resistance element on the base substantially concentric with the shaft; a metal collector member on thebase inside the arcuate resistance element; a driver carried by the shaft and overlying the base; a contactor having oppositely extending long and short spring arms, the extremities of which are adapted for sliding engagement with the resistance element and metal collector member respectively; and means for connecting the contactor with the driver; said arms being so proportioned that the thrusts of the long and short arms are balanced about an axis substantially intersecting the shaft axis and lying between the inner ends of the long and short arms so that the unit pressure on the metal collector member is of greater magnitude than that applied on the resistance element; said contactor being an integral unitary stamping so that variations in thickness, temper and other factors inherent in the stock affect the long and short arms equally.

3-. As an article of manufacture, a contactor for variable resistance devices of the type used in radio control equipment, comprising: a substantially. flat attaching portion by which the contactor is connectible to a suitable carrier; and long and short spring arms extending in opposite directions from said attaching portion, the extremities of said arms being adapted to have sliding contact with a resistance element and a metal collector contact respectively, the strength of said arms being so proportioned that the thrusts of which they are capable are balanced about an axis lying in the plane of the attaching portion substantially medially between the inner ends of the long and short arms; and said contactor being an integral unitary stamping of resilient sheet metal so that variations in thickness, temper and other factors inherent in the stock affect the long and short arms equally.

4. As an article of manufacture, a contactor for variable resistance devices of the type used in radio control equipment, comprising: a flat attaching portion adapted to be mounted on a suitable carrier; a pair of arcuate spring arms extending in the same direction and toward each other from opposite ends of the attaching portion; a second pair of spring arms extending from the ends of the attaching portion in a direction opposite to that of the first named arms and also directed toward each other, said second named arms being shorter than the first named arms so that the extremities thereof which are formed to have sliding engagement with a metal surface lie closer to a linedrawn from end to end of the attaching portion than the extremities of the first named arms which are formed to have sliding engagement with a resistance element; the arms of both pairs being biased in the same direction away from the attaching portion so that their extremities can press down with a spring tension on surfaces which the arms engage, said arms being so proportioned that the downward thrust of which the long arms are capable balances the downward thrust of which the short arms are capable about an axis joining the ends of the attaching portion so that the short arms are capable of higher unit pressures than the long arms; and said contactor being an metal so that variations in thickness, temper and other factors inherent in the stock affect all of the arms equally.

5. As an article of manufacture, a contactor for variable resistance devices of the type used in radio control equipment, comprising: a single unitary stamping of resilient sheet metal, said stamping being formed with two o pposed pairs of long and short spring arms connected at their inner ends by an attaching portion, and said attaching portions of the two pairs of arms being connected by a transverse bridging member disposed in the space defined by said arms, said attaching portions being adapted to be drivingly connected with a contactor carrier, and all of the spring arms being biased in the same direction so as to project away from the carrier, and the strength of said spring arms being so proportioned that the thrust which the long arms are capable of exerting balances the thrust which the short arms are capable of exerting about an axis lying substantially medially between the inner ends of both pairs of long and short arms so that the short arms can apply higher unit pressures thanthe long arms.

6. In a variable resistance of the type used in radio receivers, a contactor assembly comprising: a carrier in the form of a disc-shaped piece of insulating material having a central hole therethrough for the reception of an operating shaft; a metal contactor overlying one face of said carrier and having a flat attaching portion; means for securing said attaching portion flatly to said face of the carrier in a manner securing said carrier and contactor against accidental disassembly; a pair of long arci'iate spring arms extending from the opposite ends of the attaching portion of the contactor with their extremities overhanging the adjacent edge of the carrier; a pair of short spring arms extending in opposite directions from the ends of said attaching portion with the extremities thereof overlying the carrier between its central hole and adjacent outer edge; said long and short spring arms being biased in the same direction away from the carrier, and said arms being so proportioned that the thrusts which the long arms are capable of balance the thrusts which e short arms are capable of about an axis substantially intersecting the hole in the carrier and lying in the. plane or the attaching portion so that higher unit pressures are obtained with the short arms than with the long arms; and said contactor being an integral unitary stamping of resilient sheet metal so that variations in thickness, temper and other factors inherent in the stock affect all of the arms qu NEWTON C. SCHELLENGER. 

